This invention relates to control apparatus for the dynamic braking of a three-phase or single-phase induction motor particularly in an arrangement in which on disconnection of the motor from the power supply a capacitor is connected to a pair of terminals of the primary windings such that self-excitation is produced.
It should be noted that the terms `three-phase` and `single-phase` are used to define only the type of supply for which a particular motor is intended without reference to the specific construction of the motor. Thus a single-phase motor will have primary windings of polyphase form and some means of phase correction, such as a capacitor, to enable operation from a single-phase supply. The polyphase windings may be arranged as for a three-phase supply, or may have their axes in quadrature. To avoid possible ambiguity, particularly as between star-connected and delta-connected motors, the terms "a pair of terminals of the primary windings" is used in this specification to refer in the context of a three-phase motor to a pair of the three terminals to which the supply is connected and in the context of a single-phase capacitor-start motor to a pair of the three terminals normally provided.
The effect of self-excitation is to produce a strong dynamic braking torque which is of short duration but sufficient to cause a significant reduction in speed. Thereafter the motor will normally coast to a standstill which is particularly undesirable either for emergency stops or for cyclic production processes in which a motor must be stopped and then reversed or restarted in the shortest time consistent with tolerable mechanical stress. Various secondary braking mechanisms, mechanical or electrical, have been proposed to eliminate coasting and it is usual to arrange that such a mechanism should take effect before the end of self-excitation. It is however difficult to provide precise control of the braking process.